Controlled uptake and release

One widespread application, which has been much researched in recent years, is the use of colloidal systems for controlled uptake and release purposes. 

Another interesting exploitation of DTE derivatives, in this case bearing A-methylpyridinium pendant groups, is based on the fact that the closed isomer behaves as an electron acceptor, while the open state formed by visible light irradiation does not. Thus, a molecularly imprinted matrix is formed by cross-linking yia electropolymerization of thioaniline-functionalized Au nanoparticles in the presence of the closed photoisomer. The system is built in such a way that the closed photoisomer is then released to the electrolyte solution, so that the cyclic reduction and oxidation of the bridging units results in the reversible electrochemically controlled uptake and release of the closed photoisomer. The system finds a number of applications. ... 

You, Y.-Z., Kalebaila, K.K., Brock, S.L., and Oupicky, D. (2008) Temperature-controlled uptake and release in PNIPAM-modified porous silica nanoparticles. Chem. Mater., 20 (10), 3354-3359. 

Stimuli-responsive materials have attracted enormous interest in the fields of polymer, supramolecular, and colloidal science over the last few decades [1-5]. It can be anticipated that stimuli-sensitive systems in general and switchable polymers in particular will continue to be a thriving branch of chemistry. New applications can be envisioned, for instance, for controlled uptake and release, fine-tuning of mechanical properties, or actuating purposes. 

As reactive P is transported through the terrestrial system, it is assimilated into plants and subsequently into the rest of the biosphere (2). Although many elements are required for plant life, in many ecosystems P is the least available and, therefore, limits overall primary production (Schindler, 1977 Smith et al., 1986). Thus, in many instances the availability of P influences or controls the cycling of other bioactive elements. When organisms die, the organic P compounds decompose and the P is released back into the soil-water system. This cycle of uptake and release may be repeated numerous times as P makes its way to the oceans. 

Clay minerals are important to the crustal-ocean-atmosphere fectory, not just for their abundance, but because they participate in several biogeochemical processes. For example, the chemical weathering reactions responsible for their formation are accompanied by the uptake and release of cations and, thus, have a large impact on the chemical composition of river and seawater. This includes acid/base buffering reactions, making clay minerals responsible for the long-term control of the pH of seawater and, hence, of importance in regulating atmospheric CO2 levels. 

Nakamura, K., Maitani, Y., Lowman, A.M., Takayama, K., Peppas, N.A., and Nagai, T., Uptake and release of budesonide from mueoadhesive, pH-sensitive copolymers and their application to nasal delivery, J. Control. Rel. 61 329-335 (1999). 

This cycle plays a major role in controlling the rates of both glucose uptake and release by the Uver. It is the changes in concentration of glucose in the Uver that determine the direction and rate of glucose metabolism (described above). 

A. D. Cherrington (1999). Banting Lecture 1997. Control of glucose uptake and release by the liver in vivo. Diabetes 48 1198. 

Calcium levels are believed to be controlled in part at least by the uptake and release of Ca2+ from mitochondria.172"174 The capacity of mitochondria for Ca2+ seems to be more than sufficient to allow the buffering of Ca2+ at low cytosol levels. Mitochondria take up Ca2+ by an energy-dependent process either by respiration or ATP hydrolysis. It is now agreed that Ca2+ enters in response to the negative-inside membrane potential developed across the inner membrane of the mitochondrion during respiration. The uptake of Ca2+ is compensated for by extrusion of two H+ from the matrix, and is mediated by a transport protein. Previous suggestions for a Ca2+-phosphate symport are now discounted. The possible alkalization of the mitochondrial matrix is normally prevented by the influx of H+ coupled to the influx of phosphate on the H - PCV symporter (Figure 10). This explains why uptake of Ca2+ is stimulated by phosphate. Other cations can also be taken up by the same mechanism. 

Cyclic uptake and release of Ca2+ from the extracellular medium occur during mitosis in Physarum pofycephalum, and correlate with specific structural and kinetic events in the mitotic nuclei.442 The membrane system in the mitotic apparatus in Haemantkus endosperm cells functions in the localized release of Ca2+, so regulating the events of mitosis.443 It is known that calcium exerts effects on the stability of spindle microtubules. An alternative view is that free magnesium concentration acts as the fundamental regulator of the cell cycle.444 Tubulin polymerization depends on the presence of magnesium and the absence of calcium, and control of the Ca2+/Mg2+ ratio is relevant to spindle assembly. 

Although bone is not considered a major calcium sensing organ in humans, the cells of bone tissue control over 99% of the human body s calcium content. The principal calcium sensors that regulate bone calcium uptake and release are in the parathyroid glands. Bone function is also modified by vitamin D and by calcium transport in the kidney and intestine. These indirect mechanisms of controlling bone calcium metabolism are beyond the scope of our considerations here. In spite of processing... 

Summary - Many diverse novel compounds that inhibit different platelet functions show great promise, not only for potential anti-thrombotic agents, but also for more specific effects on prostaglandin and/or thromboxane A2 synthesis, and serotonin or calcium uptake and release. Many active compounds can be used as tools in the search toward a more complete understanding of the physiologic interactions of the hemostatic mechanisms. This better understanding would lead to the development and use of more potent and selective synthetic compounds in the inhibition of platelet aggregation and fibrin formation, and in the enhancement of fibrinolysis for the control of both arterial and venous thrombosis. It is hoped that some of these new compounds will be evaluated clinically in the near fu ture. 

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